1. Field of the Invention
The present invention relates to a seed layer for a nickel oxide (NiO) pinning layer for increasing the magnetoresistance of a spin valve sensor and more particularly to a seed layer between the first gap layer and the nickel oxide (NiO) pinning layer of a bottom spin valve sensor for increasing the magnetoresistance of the sensor.
2. Description of the Related Art
A spin valve sensor is employed by a read head for sensing magnetic fields on a moving magnetic medium, such as a rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive first spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90.degree. to an air bearing surface (ABS) which is an exposed surface of the sensor that faces the magnetic medium. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetic moment of the free layer is free to rotate in positive and negative directions from a zero bias point position in response to positive and negative magnetic fields from a moving magnetic medium. The zero bias position is the position of the magnetic moment of the free layer when the sensor is in a quiescent state, namely when the sense current is conducted through the sensor without any magnetic field incursions from a rotating magnetic disk. The magnetic moment is preferably parallel to the ABS in the quiescent state of the sensor. If the magnetic moment of the free layer is not substantially parallel to the ABS in the quiescent state, there will be read signal asymmetry upon the occurrence of positive and negative magnetic field incursions from a rotating disk.
The thickness of the spacer layer is chosen to be less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons are scattered by the interfaces of the spacer layer with the pinned and free layers. When the magnetic moments of the pinned and free layers are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. Changes in scattering changes the resistance of the spin valve sensor as a function of cos .theta., where .theta. is the angle between the magnetic moments of the pinned and free layers. A spin valve sensor has a significantly higher magnetoresistance than an anisotropic magnetoresistive (AMR) sensor. For this reason it is sometimes referred to as a giant magnetoresistive (GMR) sensor.
The location of the bias point on the transfer curve is influenced by four major forces on the free layer, namely a ferromagnetic coupling field (H.sub.FC) between the pinned layer and the free layer, a demag field (H.sub.demag) from the pinned layer, sense current fields (H.sub.SC) from all conductive layers of the spin valve sensor except the free layer, and the influence of an AMR effect. The influence of the AMR effect on the bias point is the same as a magnetic influence and can be defined in terms of magnitude and direction.
A bottom spin valve sensor typically employs a nickel oxide (NiO) pinning layer for pinning the magnetic moment of the pinned layer perpendicular to the ABS. The pinning layer is formed directly on the first gap layer of alumina (Al.sub.2 O.sub.3) and the pinned layer is formed directly on the pinning layer. Subsequent layers formed are the spacer layer, the free layer, the second gap layer and the second shield layer. At this stage the spin valve sensor has a magnetoresistance which is dR/R where R is the resistance of the sensor and dR is the change in resistance of the sensor upon the application of the applied field. Subsequently, the write head is formed on the spin valve sensor. In the formation of the write head the multiple photoresist layers of the insulation stack are hard baked 225.degree. to 250.degree. for 6-11 hours. This hard baking reduces the aforementioned magnetoresistance of the sensor. The amount of reduction determines the thermal stability of the sensor.
Efforts continue to build spin valve sensors with high magnetoresistance that survive the hard baking cycles of the write head. High magnetoresistance equates to increased sensitivity of the spin valve sensor to magnetic flux incursions from a rotating disk. Another consideration, however, is the ferromagnetic coupling field (H.sub.C) between the pinned layer and the free layer. It is desirable to minimize the ferromagnetic coupling field since the ferromagnetic coupling field affects the bias point of the sensor. A ferromagnetic coupling must be counterbalanced by some other magnetic field in order to achieve a zero bias point. Sense current fields exerted on the free layer must also be counterbalanced. As the search continues to increase the magnetoresistance it is important that the spin valve sensor not be degraded by some other factor, such as a high ferromagnetic coupling between the pinned and free layers.